The Democratic People's Republic of Korea (DPRK) announced it conducted a second nuclear test on 25 May 2009. Within hours of the test, and before the official DPRK announcement, several organizations, including the U.S. Geological Survey National Earthquake Information Center and the Comprehensive Test Ban Treaty Organization (CTBTO), reported a seismic signal in the magnitude range 4.5 to 4.7 near the vicinity of the 2006 DPRK nuclear test (CTBTO Press Centre, 2009a). However, the International Monitoring System of the CTBTO did not detect radioactive noble gases that would confirm the test of a nuclear-device (Clery, 2009).

Figure 2.18:
Map of the Yellow Sea / Korean Peninsula region. The North Korea explosion is identified with a star and the stations with triangles. The region is outlined in the global inset map.

Figure 2.19:
Source-type plot with various solutions and their associated fit percent. Note that the Best-DC solution is at the center ([,k] = [0,0]) and the Best-dev solution is along the abscissa (k = 0). Also plotted are results for explosions at the Nevada Test Site (NTS) from Ford et al. [2009a] with their associated 95% error ellipses. Error ellipses for the North Korea test are smaller than the plotted symbol.

In our previous work (Ford et al., 2009a) we showed it was possible to identify explosions at the Nevada Test Site with a full moment tensor inversion of seismic waveforms from nearby stations in the Western US. In this contribution, we show that the best-fit source for the 2009 Memorial Day event near Kimchaek, North Korea is dominantly isotropic, which is consistent with an explosion. A more detailed description of this work is found in Ford et al. (2009b)

Three component waveform data from Global Seismograph Network (GSN) and China Digital Seismograph Network (CDSN) stations MDJ, INCN, BJT and HIA along with station TJN from the Ocean Hemisphere Project Seismological Network (Figure 2.18) are instrument corrected, integrated to displacement, and band-pass filtered using an order acausal Butterworth filter with corners at 0.02 and 0.10 Hz. Green's functions were computed using a model appropriate for the region. The Green's functions were aligned with the data based on a location and origin time of 00:54:43.38, 25 May 09, 41.2986/129.0694 (D. Dodge, unpublished data, 2009), and timing shifts of less than 5 seconds (maximum half-cycle recommended by Ford et al. (2009a)) were employed to maximize the fit to the data. A source depth of 600m was initially assumed. Explosion, earthquake (double-couple or DC), deviatoric, and full moment tensor solutions were evaluated The explosion and DC solutions were obtained with a grid-search to find the best-fitting parameters, whereas the deviatoric and full moment tensor solutions were calculated with a least-squares linear inversion.

As Figure 2.19 shows, the pure explosion model fits the data with a variance reduction of 75% and yields an isotropic moment of dyne-cm (4.1; all seismic moment values are calculated with the method of Bowers and Hudson (1999)). In contrast, the pure DC earthquake solution fits the data much worse at 52% with = dyne-cm (4.4). The fact that the single degree of freedom explosion model fits so much better than the four degree of freedom DC model is highly significant and indicates that such a comparison can be a useful discriminant. The strike, rake, and dip of the best-fit DC is 50, -85, and 10, respectively. Such a steep dip-slip mechanism is unusual for an earthquake. Of all sources calculated by the Global CMT Project (http://www.globalcmt.org/) less than 1.6% have dips of less than 10. This type of information may be used as an additional indication of anomalous sources. The DC overpredicts the Love wave amplitude on the transverse components at almost all stations and underpredicts the Rayleigh wave amplitudes, especially at station INCN. The pure explosion does not produce Love waves, and therefore the actual source is a composite.

The full moment tensor inversion fits the data at 81% and yields an isotropic moment of dyne-cm, and a total moment of dyne-cm (4.5). The deviatoric moment tensor inversion fits the data at 80% and a total moment of dyne-cm (4.3). If the deviatoric source is decomposed to a compensated linear vector dipole (CLVD) and DC sharing the same principal axes, then the source is 70% CLVD. The similarity in fits between the dominantly CLVD deviatoric source and dominantly isotropic full moment tensor shows that at shallow depths, a vertical CLVD mechanism can effectively mimic an explosion at the distances and periods analyzed here. The full moment tensor isotropic moment is two times larger than the pure explosion, indicating that the compound source of the full moment tensor solution (DC+CLVD+Isotropic) required to fit the Love waves also modifies the Rayleigh waves, causing the isotropic component to increase in order to compensate.

The best-fitting sources are far from the center, indicating that the source is anomalously non-DC (Figure 2.19). Along with the best-fitting sources the solutions and their 95% error ellipses for explosions at the Nevada Test Site (NTS) and earthquakes in the Western US from Ford et al. (2009a) are also plotted. The best-fit full moment tensor plots in the same region as the explosions, close to the solution for the 2006 North Korean test, and away from the earthquake population. The error ellipses for the best-fit sources from this study are all smaller than the symbol used to plot the solutions due to the very high signal-to-noise of the event records.

Unlike earthquake inversions, the isotropic radiation of a predominantly explosive source does not allow constraint of the source depth by comparing fits at different depths at the frequencies examined here (e.g. Ford et al., 2009a). Event locations put the source at less than 1 km, so the results discussed above assume a source depth of 600 m. Ford et al., (2009b) examines the sensitivity of isotropic moment with source depth

Modeling of low-frequency, regional distance waveforms identifies the Memorial Day event in Kimchaek, North Korea as decidedly non-tectonic with the best-fit model dominated by an explosion source. While the source type is well determined to be non-DC, the isotropic moment of the full moment tensor inversion has some uncertainty and the is between 4.4 and 4.6. Comparison of pure explosion and pure double-couple models indicate that the simpler explosion model fits the waveform data substantially better than the higher degree of freedom DC model, where the isotropic moment of the explosion model is dyne-cm (4.1). However, there are Love waves observed at several stations indicating that the source must have some non-isotropic component. Possible causes of the tangential displacement are additional tectonic sources, tensile failure at depth, and anisotropic propagation.